It is impossible to enjoy idling thoroughly unless one has plenty of work to do. There is no fun in doing nothing when you have nothing to do. Wasting time is merely an occupation then, and a most exhausting one. Idleness, like kisses, to be sweet must be stolen. - Jerome K. Jerome

The most effective teaching methods involve giving students the tools to be able to learn how to learn. Most learning will be done on a student's own through exploration, even if much of it is passive.

That's where video games come in. Legend of Zelda may not teach you Mayan history, it might not show you, directly, how to do algebra, but it develops problem solving and creative thinking skills in fairly complex ways that will make a student's job in learning those things FAR easier. Zelda isn't even an "education game" but its innate problem solving is more involved that almost any story problem you'll encounter in HS, and kids play Zelda in grade school. The problem is, it's not easilly quantifiable because there are no hard-and-fast facts being learned, but as I said, fact learning is one of the least inefficient educational methods. Sure, facts must be taught, but there should be much less emphasis on fact learning and more emphasis on critical thinking skills.

Meanwhile, over the course of Zelda, or even an adventure FPS, RPG, or most other modern games, you're reading a lot of on-screen text, you're doing mathmatical computation for stats, puzzles, and the like... and all surrounded by various time limits that act as drill. And to top it off, it's fun and doesn't FEEL like work. What more could an educator ask for?